Tracer experiments on the dispersion of plumes over terrain of major surface roughness

Vogt, K. J.; Geiß, H.

Book/Report

FZJ-2018-00277

Tracer experiments on the dispersion of plumes over terrain of major surface roughness

Vogt, K. J.FZJ* ; Geiß, H.FZJ*

1974
Kernforschungsanlage Jülich, Verlag Jülich

Jülich : Kernforschungsanlage Jülich, Verlag, Berichte der Kernforschungsanlage Jülich 1131, 19, [15] p. (1974)

Please use a persistent id in citations: http://hdl.handle.net/2128/16472

Report No.: Juel-1131-ST

Abstract: In the Jülich Nuclear Research Center, field experiments on the atmospheric dispersion of plumes by turbulent diffusion have been carried out in order to measure the diffusion parameters required for environmental exposure calculations. For that purpose, test aerosols, tracered with Cu 64, have been released under different weather conditions from the 50 m and 100 m platforms of a meteorological tower, and the ground level concentration distribution below the plume has been measured up to a source distance of 10 km with a grid of 100 sampling stations. The results of more than 40 experiments show that, compared with the predictions based on Pasquill's system, the source distance of the ground level concentration maximum is reduced by a factor of 2 to 5, depending on the diffusion category. This discrepancy can be explained by differences in the surface roughness, which is not taken into account in the determination of the diffusion category by systems based on synoptic meteorological observations. A neglection of the increased mechanical turbulence over rough terrain results in an underestimation of the environmental exposure for small source distances and an overestimation for larger distances In order to enable the application of the results of diffusion experiments to other sites, it is necess~ry to determine the diffusiori categories on the basis of meteorological parameters directly correlated with the local turbulence and, hence, with the surface roughness of the terrain. The measured diffusion parameters have been used to calculate the short-time diffusion factors for the various exposure pathways.The long-time diffusion factors, representing the annual concentration distributions in the different sectors of the windrose, have been computed by superposition of the short-time concentrations according to the 3-dimensional frequency distribution of wind direction, wind velocity and diffusion category, which have been measured in Jülich for more than five years. The most important result is involved in the fact that, when using Pasquill's diffusion parameters for sites featuring a surfaceroughness similar to Jülich, the maximum of the long-time distribution curve is being underestimated by about the factor 5 as against the values obtained in the course of the Jülich diffusion experiments at the usual emission heights. In those cases where the 3-dimensional frequency distribution at a location has not been measured, it is possible to determine its long-time diffusion factors by approximation; this is done by classification of such location into a base siting category, for which diffusion-climatological statistics are available, and by superposition of the wind direction distribution of this specific site on the distribution functions computed for these statistics.

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